Why do some planets have rings? Why do none of the
inner planets have any?

Question Date: 2002-09-24

Answer 1:

This is a very good question, one that is still a
topic of scientific research. Here is the basic
theory:The solar system formed from a cloud of
cold gas that collapsed due to gravity. A big
glob of stuff formed in the center and eventually
became the sun. Meanwhile, some of the cloud
material orbited around the proto-sun and
flattened into a disk. In the disk, some matter
came together to form small planetoids that
slowly
grew. The matter that was closer to the center
was also warmer so only the more dense stuff such
as metals and rocks combined together to form
planets; the warm gas was moving too fast to get
caught. Farther away, everything was cooler so
gases like hydrogen and helium could also get
sucked up by the new planets. So the planets
closer to the sun (Mercury, Venus,Earth, and Mars)
are small and rocky while the ones farther away
(Jupiter, Saturn, Uranus, and Neptune) are big gas
giants.

Because it was cooler farther away
from the sun it seems that it was easier for the
big gas giants to also form moons (in fact there is
quite a controversy regarding how the Earth and
Mars got their satellites). As it turns out, these
moons probably help keep trapped material that the
planet has caught in rings instead of flying
away or crashing into the planet. In addition,
the rings seem to be partly made of frozen gases
which don't exist closer to the sun. So the
bottom line is that the farther away gas giants
are much more likely to be able to form and keep
rings than the inner rocky planets.